Space conditioning systems are used to maintain desirable levels of temperature, humidity, and fresh air inside of buildings, vehicles, and the like. Of particular importance in the proper operation of such a system is maintaining proper humidity levels. In addition to imposing discomfort on occupants, high humidity levels can lead to the growth of molds, fungi, and other non-desirable organisms. These concerns lead to the ability of the space conditioning system to remove humidity from the air (referred to as latent heat load) being a major design consideration.
At the same time, concerns over poor indoor air quality caused by inadequate air exchange have led to the desire for higher fresh air ventilation rates. When the outside relative humidity is elevated, such high fresh air ventilation rates coupled with the desire for low internal humidity levels lead to a high latent heat load on the space conditioning system.
The typical method by which large amounts of humidity are removed is to reduce the temperature of the air being supplied to the building to a level at which the saturation pressure of water vapor is substantially below the actual partial pressure of the water vapor in the air. In doing so, the excess water vapor will condense to liquid water, and can be removed from the air. This results in the dew point temperature of the supply air being fixed at about that temperature to which the air has been reduced.
Oftentimes, however, the desirable dew point temperature is lower than the desirable dry-bulb temperature at which the supply air is to be delivered for occupant comfort. In order to meet both the humidity and the temperature requirements, the supply air is often reheated to a desirable dry-bulb temperature after the excess humidity has been removed. Significant energy can be expended by first cooling the supply air to remove the latent head, and then reheating the air to an acceptable temperature. In order to improve the energy efficiency of a space conditioning system, it is desirable to recover some of the latent heat load that is removed for humidity control as the heat source for reheating the air to an acceptable temperature.